1. Field of the Invention
This invention relates to a method for temporary storage of wafers made of silicon (hereinafter referred to as "wafer") in the process of production. More particularly, this invention relates to a method for enabling wafers in process to be stored between the component processes without exposing the surfaces thereof to contaminant, in the fabrication of mirror wafers by the treatment of wafers cut from a single crystal silicon rod in various sequential processes.
2. Description of the Prior Art
In the fabrication of mirror wafers from wafers cut from a single crystal silicon rod, the said wafers are generally treated in the processes of chamfering, lapping, washing, etching, prewashing for annealing, and annealing in the order mentioned and thereafter polished until mirror finish.
When the treatment includes an additional process of sandblasting, the process is interposed between the etching and the annealing process mentioned above and the annealing process is followed by a washing after sandblating and a prewashing for annealing.
The workshop engaging in the manufacture of wafers is at times compelled to put wafers in process to protracted storage between the component processes mentioned above to suit the convenience of the job system. At a factory operated on the five-day week system, for example, the durations of such storage possibly totals about 60 hours.
Heretofore, in the lapping through the annealing process mentioned above, this protracted storage has inevitably exposed the surfaces of wafers to contaminant as specifically described hereinbelow in spite of very severe control of the conditions of the storage. For this reason, restrictions have been frequently imposed on the pertinent processes possibly to the extent of rendering difficult the improvement of the operation rate of the plant or the productivity of the operation.
To shorten the storage time, it becomes necessary for the workshop to install a fairly large sum of facilities and arrange workers for attending such facilities or adopt a 24-hour day 7-day week job system or it becomes inevitable for the workers to suffer the burden of the waiting times (which are necessary for shortening the storage time of wafers ) between the component processes of the treatment of wafers. Further, the fact that the ratio of the waiting time to the working time cannot be ignored entails the problem that the job system is rationalized only with difficulty.
Here, various sorts of contaminant which occur on the surfaces of wafers will be described. The wafers in process are washed before and after each process, then either immersed in purified water or dried, and stored as placed in an atmosphere of clean dry air (degree of cleanliness 100 or over) at normal room temperature. In spite of all these efforts, the surfaces of wafers yield to the sorts of contaminant which are shown in Table 1 below.
TABLE 1 ______________________________________ Etching/- washing Pattern of process (with Conditions of Start of surface liquid) storage contamination contamination ______________________________________ After Purified After 30 contamination etching water minutes like aurora After post- Clean dry After 8 contamination washing of air hours in bluish etching white color After post- Clean dry After 8 contamination washing of air hours in bluish sandblasting white color After pre- Clean dry After 5 Basket washinng for air minutes contamination annealing ______________________________________
In Table 1, the ratings of surface contamination are obtained visually under a focusing lamp. To be specific, in the parts sustaining "contamination like aurora" and "basket contamination", the occurrence of coarse surface, protrusions, and pits was confirmed. It is inferred that these surface defects were caused by fine etching with residual corrosive chemical liquids. The "contamination in bluish white color" was such contamination as expanded in the form of a strip in the outer through the outer peripheral part of a given wafer. It is inferred that this contamination was caused by not only the action of the chemical liquids mentioned above but also the conditions of the atmosphere (temperature and humidity) used for the storage. The "basket contamination" was such contamination as occurred in the parts where the ribs of a basket used for storing and conveying wafers contacted with the wafers. Though the cause therefore remains yet to be elucidated, it is inferred that this contamination was caused by the chemical liquids which persisted at the sites of the contact mentioned above.
Of the various chemical liquids which are usable in the processes of treatment indicated in Table 1, that which is usable in the processes of etching is either the aqueous solution of hydrofluoric acid, a mixed acid of hydrofluoric acid and nitric acid, or a mixed acid of these two acids plus acetic acid or the aqueous solution of caustic soda or caustic potash, for example.
The chemical liquid which is usable in the processes of washing is the aqueous solution of such an acid as hydrofluoric acid, hydrochloric acid, or citric acid or the aqueous solution of such an alkali as caustic soda, caustic potash, or ammonia, optionally additionally containing therein a small amount of hydrogen peroxide or a surfactant.
Even when the washing with purified water is repeated again, the wafers already dried are stored in their unmodified form in a clean bench filled with highly clean atmosphere, or the wafers washed with purified water are stored in their unmodified form in purified water on the assumption that the contamination of surfaces of wafers are caused by the aforementioned chemical liquids which are not perfectly removed by the subsequent washing with purified water or by drying or by contaminant remaining in the atmosphere used for the storage of wafers, the contamination caused on the surfaces of waters after the elapse of a certain length of time can not be perfectly removed.
Though the aforementioned repetition of the washing with purified water is recognized to be veritably effective in decreasing the frequency of the contamination of surfaces of wafers, this method entails extra time and labor and nevertheless offers no decisive solution to the problem of contamination.